1. Field of the Invention
The present invention relates to switching power supplies, or power converters, and more particularly to very small and low cost switching power converters whose structural configuration permits automated assembly and elimination of the conventional transformer.
2. Description of the Prior Art
The consumer electronics revolution has resulted in a wide array of small, highly portable devices that typically run on batteries. For example, small Japanese televisions, radios, and CD players almost universally are supplied in the United States with earphones for private listening and AC power adaptors to save battery drain. If the device does not come with an AC adaptor, one is usually easily available, either from the original manufacturer or an after market supplier.
Two basic kinds of AC adaptors are ubiquitous, the linear type and the switching type. The linear type uses large 60 Hz transformers. FIG. I is a schematic of a typical prior art linear type AC adaptor which incorporates a 60 Hz transformer. The switching type uses a circuit to chop incoming power into high frequency pulses and can use very small and light transformers. FIG. 2 is a schematic of a relatively expensive prior art switching type power supply which feeds back voltage, current and/or power, in a closed loop control circuit, to regulate its output. Below ten watts, it has been more cost effective for manufacturers to supply the linear type AC adaptor, even though the 60 Hz stepdown transformers can get quite bulky. At about ten watts, a cost-to-produce cross-over point is reached, and the more sophisticated switching type AC adaptors become cost effective. When switching type AC adaptors are used, the consumer benefits from the reduced size and weight of the unit that must be plugged into a wall plug.
Cost is a critical factor in being able to compete in the consumer electronics market. High performance is often not as important to consumers as low purchase cost. So manufacturers often choose to supply large and bulky 60 Hz transformers in plastic housings resembling bricks for their designs, even though smaller, lighter weight and more efficient designs are possible using switching power converters. One reason the linear type AC adaptor has a lower production cost is that the transformers used in switching type AC adaptors are difficult to assemble with automated equipment. Manual labor must be employed, and that lowers volumes and increases the per unit cost. Very often, the labor component in manufacturing costs is the biggest expense in producing power supplies under two watts.
Prior art linear type AC adaptors have efficiencies that reach only as high as fifty to sixty percent. A lot of power gets thrown-off as heat. These low efficiencies limit the power range of this type of power supply because the waste heat may make the unit too hot to handle, or dangerous to operate in certain situations.
Thus the prior art forces the designer or manufacturer of consumer electronic equipment to be faced with a choice between a heavy, large and inefficient linear power supply (which has a 60 Hz transformer) but which is inexpensive to produce, or a relatively light-weight, small-sized and very efficient switched mode power supply that is expensive to produce. Generally, the choice is made in favor of the lower cost, large, heavy and inefficient power supply.
Therefore a need exists for an AC adaptor technology that can provide the performance advantages of a switching type power supply, such as high efficiency, small size and light weight, while also providing the cost advantages of the 60 Hz transformer type power supplies. The present invention solves the problem of the expense of manufacturing a switching type power transformer by a novel method described below in detail.